My invention relates to multiple plate type clutches. Such clutches often are used in heavy-duty, manually-controlled truck transmissions, an example of which I have shown in FIGS. 1 and 2. A heavy intermediate plate is used in the construction of FIGS. 1 and 2 between two friction disc assemblies. A pressure plate is adapted to engage one of a pair of friction discs, and the force applied by that pressure plate is transferred to the intermediate plate and to the second friction disc. In an arrangement such as that shown in FIGS. 1 and 2, the reaction force of the pressure plate occurs at an annular friction surface on the flywheel of the engine, usually an internal combustion engine.
The intermediate plate typically is driven by means of lugs in the flywheel or by dogs on its margin that engage slots in the flywheel. The force of the vibrations in the internal combustion engine cause noise in the clutch assembly at low speeds due to torsional vibrations. The vibrations eventually cause wear of the drive pins, the lugs, the dogs or the drive slots that connect the intermediate plate with the flywheel.
In some prior art constructions a spring strap drive system is used for supporting and centering the intermediate plate with respect to the flywheel. The straps are situated with a generally tangential disposition with respect to the axis of the clutch so that the centerline of each strap defines a secant with respect to the outer peripheral extremities of the clutch. The use of straps may eliminate some of the noise problem caused by torsional vibrations developed by the engine, but the straps do not allow for floating movement of the intermediate plate.
The drive straps are designed also to bias the center intermediate plate to a position that will allow both friction clutch disc assemblies to rotate freely on release, but this position of the intermediate plate is of necessity a design compromise between the position that is optimum for a new clutch and the position that is optimum for a worn clutch. A worn clutch in such an arrangement would require extra travel to permit operation without frictional drag of the friction surfaces.
If the intermediate plate, which is refered to also as the drive plate, is machined during servicing following extended operation of the truck, the adjusted positions of the friction discs with respect to the clutch plates is not an optimum and friction free release of the clutch is not possible.
In other prior art constructions friction pins control the lift of the intermediate plate by using a lever system that permits a predetermined lift to release the front disc and the intermediate plate. The friction pins have a sliding resistance that exceeds the drive strap bias. The friction pins then prevent further lift of the intermediate plate. Additional clutch release travel then releases the rear disc. The friction pins are adapted to reset themselves each time the clutch is engaged as the pins respond to pressure against the flywheel. The pins thus reset for every increment of wear on the front friction disc. This system has the disadvantage of being susceptible to debris buildup on the friction pin surfaces, which can cause considerable variations in clamping load on the front disc.